Landfill Gas Purification Method and System

ABSTRACT

The present disclosure provides a method for separating and purifying a landfill gas stream. In one embodiment, the method includes a step of collecting a crude landfill gas stream which includes at least methane, carbon dioxide, oxygen, and nitrogen gases. The method also includes a step of separating the crude landfill gas stream into at least an intermediate landfill gas stream and a first waste gas stream using a membrane separation unit, wherein the intermediate landfill gas stream is enriched in methane and depleted in carbon dioxide relative to the crude landfill gas stream. The method further includes a step of separating the intermediate landfill gas stream into at least a final landfill gas stream and a second waste gas stream using a pressure swing adsorption separation unit, wherein the final landfill gas stream is enriched in methane and depleted in carbon dioxide, oxygen, and nitrogen relative to the intermediate landfill gas stream. In some embodiments. VOCs in the waste gas streams are destroyed using a thermal oxidizer.

FIELD

This disclosure relates to the separation of a mixed gas stream. Moreparticularly, this disclosure relates to a method and system for theseparation and purification of methane from landfill gases.

BACKGROUND

As household and other wastes gradually decompose in commerciallandfills, significant quantities of gases are generated asdecomposition products. These include hydrocarbons, particularlymethane, as well as carbon dioxide, nitrogen, water vapor, and othergases and hydrocarbons. If captured and recovered from the landfillsite, the methane generated during landfill decomposition represents apotentially valuable energy or feedstock source, particularly if themethane is refined or purified in such a manner as to meet the qualityrequirements to be accepted for delivery in natural gas pipelines tocustomers as a substitute for natural gas obtained from drilled naturalgas wells.

To be combusted and used as an energy or feedstock source, however, themethane must first be separated from the other landfill gases andsubstantially purified. Therefore, there is a need for improved methodsand systems to separate landfill gases, particularly methane, from othercomponents of landfill gas streams so as to provide a substantiallypurified methane stream which may be either combusted and used as anenergy source or used as a feedstock for the manufacture of otherchemicals or substances.

In a first aspect, the present disclosure provides a method forseparating and purifying a landfill gas stream. In one embodiment, themethod includes a step of collecting a crude landfill gas stream whichincludes at least methane, carbon dioxide, oxygen, and nitrogen gases.The method also includes a step of separating the crude landfill gasstream into at least an intermediate landfill gas stream and a firstwaste gas stream using a membrane separation unit, wherein theintermediate landfill gas stream is enriched in methane and depleted incarbon dioxide relative to the crude landfill gas stream. The methodfurther includes a step of separating the intermediate landfill gasstream into at least a final landfill gas stream and a second waste gasstream using a pressure swing adsorption separation unit, wherein thefinal landfill gas stream is enriched in methane and depleted in carbondioxide, oxygen, and nitrogen relative to the intermediate landfill gasstream.

In certain embodiments according to the present disclosure, the crudelandfill gas stream preferably includes from about 45 to about 55 molepercent methane, from about 20 to about 40 mole percent carbon dioxide,and from about 5 to about 20 mole percent nitrogen. The final landfillgas stream preferably includes at least about 96 mole percent methane,no more than about 1 mole percent carbon dioxide, and no more than about4 mole percent nitrogen.

In certain embodiments according to the present disclosure, the firstwaste gas stream includes volatile organic compounds, and the methodpreferably also includes a step of treating the first waste gas streamin a thermal oxidizer unit to substantially destroy the volatile organiccompounds therein so that the first waste gas stream may be releasedinto the atmosphere The second waste gas stream may also includevolatile organic compounds, in other embodiments according to thepresent disclosure, and the method preferably also includes a step oftreating the second waste gas stream in a thermal oxidizer unit tosubstantially destroy the volatile organic compounds therein so that thesecond waste gas stream may be released into the atmosphere.

In some embodiments according to the present disclosure, at least aportion of the crude landfill gas stream is separated into a firstrecycle gas stream using the membrane separation unit, and this firstrecycle gas stream is recombined with the crude landfill gas stream. Incertain embodiments, this first recycle gas stream is preferably fromabout 30 to about 40 mole percent of the crude landfill gas stream.

In certain other embodiments according to the present disclosure, atleast a portion of the intermediate landfill gas stream is separatedinto a second recycle gas stream using the pressure swing adsorptionseparation unit, and this second recycle gas stream is recombined withthe crude landfill gas stream. In certain embodiments, the secondrecycle gas stream is preferably from about 17 to about 22 mole percentof the intermediate landfill gas stream.

In still other embodiments according to the present disclosure, thecrude landfill gas stream may also include hydrogen sulfide gas or othercompounds of sulfur. In these embodiments, the method preferably alsoincludes a step of removing at least a portion of the hydrogen sulfidegas or other compounds of sulfur from the crude landfill gas streambefore the crude landfill gas stream is separated using the membraneseparation unit.

In certain embodiments according to the present disclosure, the crudelandfill gas stream further includes oxygen, water vapor, and volatileorganic compounds and wherein the final landfill gas stream is depletedin oxygen, water vapor, and volatile organic compounds relative to thecrude landfill gas stream.

In another aspect, the present disclosure provides a system forseparating and purifying a landfill gas stream. In one embodiment thesystem includes a feed gas conduit in flow communication with a crudelandfill gas stream which includes at least methane, carbon dioxide,oxygen, and nitrogen gases. The system also includes a membraneseparation unit in flow communication with the feed gas conduit forseparating the crude landfill gas stream into at least an intermediatelandfill gas stream and a first waste gas stream, wherein theintermediate landfill gas stream is enriched in methane and depleted incarbon dioxide relative to the crude landfill gas stream. The systemfurther includes a pressure swing adsorption separation unit in flowcommunication with the membrane separation unit for receiving theintermediate landfill gas stream therefrom and separating theintermediate landfill gas stream into at least a final landfill gasstream and a second waste gas stream, wherein the final landfill gasstream is enriched in methane and depleted in carbon dioxide, oxygen,and nitrogen relative to the intermediate landfill gas stream.

The system may also include a thermal oxidizer unit. In certainembodiments according to the present disclosure, the thermal oxidizerunit is preferably in flow communication with the membrane separationunit for receiving the first waste gas stream, which may includevolatile organic compounds. The thermal oxidizer unit substantiallydestroys the volatile organic compounds in the first waste gas stream sothat the first waste gas stream may be released into the atmosphere. Incertain other embodiments according to the present disclosure, thethermal oxidizer unit is preferably in flow communication with thepressure swing adsorption unit for receiving the second waste gasstream, which may also include volatile organic compounds. The thermaloxidizer unit substantially destroys the volatile organic compounds inthe second waste gas stream so that the second waste gas stream may bereleased into the atmosphere.

In certain embodiments according to the present disclosure, the systemalso includes a recycle conduit in flow communication with at least themembrane separation unit. At least a portion of the crude landfill gasstream is preferably separated into a first recycle gas stream using themembrane separation unit and this first recycle gas stream is recombinedwith the crude landfill gas stream via the recycle conduit. In certainembodiments, this first recycle gas stream is preferably from about 30to about 40 mole percent of the crude landfill gas stream.

In certain embodiments according to the present disclosure, the systemalso includes a recycle conduit in flow communication with at least thepressure swing adsorption unit. At least a portion of the intermediatelandfill gas stream is preferably separated into a second recycle gasstream using the pressure swing adsorption separation unit and thissecond recycle gas stream is recombined with the crude landfill gasstream via the recycle conduit. In certain embodiments, this secondrecycle gas stream is preferably from about 17 to about 22 mole percentof the intermediate landfill gas stream.

In certain embodiments according to the present disclosure, the crudelandfill gas stream may also include hydrogen sulfide gas. In theseembodiments, the system may also include a hydrogen sulfide separationunit in flow communication with the crude landfill gas stream forremoving at least a portion of the hydrogen sulfide gas from the crudelandfill gas stream before the crude landfill gas stream is separatedusing the membrane separation unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the disclosure are apparent by reference to thedetailed description when considered in conjunction with the figures,which are not to scale so as to more clearly show the details, whereinlike reference numbers indicate like elements throughout the severalviews, and wherein:

FIG. 1 is process flowchart showing a landfill gas purification systemaccording to one embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure provides a system and method for separating andpurifying a crude landfill gas stream to recover a methane componenttherefrom. The crude landfill gas stream is created as a result of thedecomposition of discarded waste materials in the landfill. After thecrude landfill gas stream is processed according to the presentdisclosure, a final landfill gas stream is obtained which is primarilymethane. The final landfill gas stream preferably meets a generalstandard or level of quality requirements and eligibility for commoncarrier natural gas pipeline transportation and may be accepted fordelivery in natural gas pipelines to be redelivered to customers eitheralone or mixed with natural gas and as such to be combusted or otherwiseused as a feedstock for manufacture of hydrocarbon products insubstantially the same manner as natural gas obtained from drillednatural gas wells may be combusted or otherwise used.

The exact composition of the crude landfill gas stream will varysomewhat from one landfill site to another. In general, however, thecrude landfill gas stream will include methane and possibly otherhydrocarbon gases, carbon dioxide, and nitrogen, as well as oxygen andwater vapor. In some instances, the crude landfill gas stream may alsoinclude hydrogen sulfide (H₂S) gas and/or volatile organic compounds(VOCs). A typical crude landfill gas stream may be composed of fromabout 45 to about 55 mole percent methane, from about 20 to about 40mole percent carbon dioxide, from about 5 to about 20 mole percentnitrogen, from about 2 to about 5 mole percent oxygen, from about 0.01to about 6 mole percent hydrogen sulfide, and from about 1 to about 2mole percent VOCs. The crude landfill gas stream is also typicallysaturated with water vapor. When initially recovered from the landfillsite, the crude landfill gas stream is typically at or below atmosphericpressure and is at ambient temperature in the gas collection system,which is typically about 140° F. or less.

In the final landfill gas stream, it is desired to provide the methanein a substantially purified form. In some embodiments of the presentdisclosure, it is particularly preferred that the methane in the finallandfill gas stream be provided as a “pipeline quality” gas product. A“pipeline quality” gas product is generally understood to mean gastreated to meet the specifications required and specified in variouspipeline tariffs or schedules filed by governmentally regulatedinterstate or intrastate common carrier natural gas pipelines anddistribution and utilities companies. The gas is typically processedprior to delivery to the pipeline to remove contaminating water,liquids, and other specified impurities. Thus, such “pipeline quality”gas in this case is a gas stream composed primarily of methane andhaving only a minimal amount of other gas components. The level of othergas components in a “pipeline quality” methane product is sufficientlylow so that the methane stream may be introduced into and transportedthrough commercial natural gas pipelines, intermixed with the naturalgas therein, and sold as the equivalent of natural gas.

Accordingly, in certain embodiments according to the present disclosure,the final landfill gas stream is preferably composed of at least about96 mole percent methane. Preferably the final landfill gas stream alsoincludes no more than about 1 mole percent carbon dioxide, no more thanabout 4 mole percent nitrogen, no more than about 1 mole percent oxygen,no more than about seven pounds of entrained water per million cubicfeet of gas, and no more than about 0.25 grains per hundred cubic feetof hydrogen sulfide (where 437.5 grains equal 1 avoirdupois ounce). Thefinal landfill gas stream also preferably has an energy value of about970 British Thermal Units (BTUs) per cubic foot of gas, as measured atstandard temperature and pressure. (Unless otherwise noted, all gasvolume measurements herein are referenced at standard temperature andpressure.) After separation and purification, the final landfill gasstream is preferably supplied at a pressure which is sufficient toovercome the existing operating pressure of the pipeline to which it isbeing delivered and to enter the transportation or distribution pipelineand at a temperature of from about 90 to about 110° F.

The crude landfill gas is initially collected via a plurality ofrecovery wells. The gas recovery wells include a plurality ofsubstantially vertical boreholes drilled or otherwise formed into theground at various positions though the landfill. The boreholes aregenerally from about 50 feet to about 150 feet in depth and have adiameter of up to about one foot. The boreholes are typically providedin a predetermined pattern such that each borehole is positioned fromabout 75 feet to about 200 feet apart from the next adjacent borehole.Within each borehole is a substantially vertical pipe, which isgenerally referred to as a well casing. Each well casing preferablyincludes a plurality of perforations along a portion of the pipe, sothat gas from the landfill can enter the casing from the landfill. Anetwork of horizontal pipes on or near the surface of the landfillinterconnects the well casings. A source of vacuum, such as vacuum pump,is connected in fluid flow communication with the piping network inorder to extract methane and other gases from the landfill via the wellcasings. The crude landfill gas stream so obtained is then supplied to apurification system 10 via a feed gas conduit 12, as shown in FIG. 1.

Once recovered from the landfill wells, a crude landfill gas stream 14may be separated and purified in a system which includes at least a feedgas conduit 12, a membrane separation unit 16, and a pressure swingadsorption unit 18. According to the present disclosure, the feed gasconduit 12 is in flow communication with the crude landfill gas stream14. The membrane separation unit 16 is in flow communication with thefeed gas conduit 12 and separates the crude landfill gas stream 14 intoat least an intermediate landfill gas stream 20 and a first waste gasstream 22. The intermediate landfill gas stream 20 is enriched inmethane and depleted in carbon dioxide relative to the crude landfillgas stream 14. The pressure swing adsorption unit 18 is in flowcommunication with the membrane separation unit 16 for receiving theintermediate landfill gas stream 20 therefrom. The pressure swingadsorption unit 18 separates the intermediate landfill gas stream 20into at least a final landfill gas stream 24 and a second waste gasstream 26. The final landfill gas stream 24 is enriched in methane anddepleted in carbon dioxide and nitrogen relative to the intermediatelandfill gas stream 20. The system 10 may include a hydrogen sulfideseparation unit 28 in flow communication with the crude landfill gasstream 14 for removing at least a portion of the hydrogen sulfide gasfrom the crude landfill gas stream 14 before the crude landfill gasstream 14 is enters the membrane separation unit 16. In addition thesystem 10 may also include a thermal oxidizer unit 30 for destroyingVOCs in the first and/or second waste gas streams 22, 26.

As noted above, if a substantial amount of hydrogen sulfide is presentin the crude landfill gas stream 14, the system 10 may also include thehydrogen sulfide separation unit 28 which removes at least a portion ofthe hydrogen sulfide gas from the crude landfill gas stream 14. Inaddition to being toxic, hydrogen sulfide gas can also be corrosive, asthose of skill in the art will appreciate. Accordingly, the hydrogensulfide separation unit 28 is preferably positioned upstream of themembrane separation unit 16 and in flow communication with the crudelandfill gas stream 14 to as to minimize the corrosive effects ofhydrogen sulfide gas upon the remaining downstream processing equipment.

In general, the use of the hydrogen sulfide separation unit 28 in thesystem 10 is preferred if the crude landfill gas stream 14 includes morethan about 5 parts per million (ppm) of hydrogen sulfide. The hydrogensulfide separation unit 28 preferably removes at least about 99 percentof the hydrogen sulfide gas present in the crude landfill gas stream 14.Thus, after treatment in the hydrogen sulfide separation unit 28, thecrude landfill gas stream 14 preferably includes no more than about 0.05ppm of hydrogen sulfide. The hydrogen sulfide separation unit 28 ispreferably sized in accordance with the flow rate of the crude landfillgas stream 14 to be treated.

The hydrogen sulfide separation unit 28 removes the hydrogen sulfide gasfrom the crude landfill gas stream 14 by introduction of the gas stream14 in to a specialized pressure vessel contacting a bed of pellets whichselectively chemically react with substantially only the hydrogensulfide gas from the crude landfill gas stream 14 while allowing theremainder of the crude landfill gas stream 14 to pass throughunaffected. The hydrogen sulfide is thus transferred to, and reactswith, the pellets which may in turn be safely disposed of following thechemical reaction.

An example of a suitable hydrogen sulfide separation unit 28 for use inaccordance with the present disclosure is the SulfaTreat system,manufactured by M-I, L.L.C, of Chesterfield, Mo. After the crudelandfill gas stream 14 has been treated in the hydrogen sulfideseparation unit 28 (if present), the gas stream is preferably compressedto a desired elevated pressure, typically about 200 psig, using asuitable gas compressor 36 in order to facilitate further processing ofthe gas downstream in the treatment system. Suitable compressors forthis purpose may be electrically powered or gas powered and are widelyknown to persons of ordinary skill in the art.

The crude landfill gas stream 14 then passes through the feed gasconduit 12 and enters the membrane separation unit 16 which is in flowcommunication with the feed gas conduit 12. In general, the membraneseparation unit 16 is used to remove a large portion of the carbondioxide from the crude landfill gas stream 14. The membrane separationunit 16 may also remove water dust and other solids, siloxanes, smallquantities of hydrogen sulfide and VOCs from the landfill gas stream aswell. In so doing, the membrane separation unit 16 separates the crudelandfill gas stream 14 into at least two gas streams, the intermediatelandfill gas stream 20 and the first waste gas stream 22. Relative tothe crude landfill gas stream 14, the intermediate landfill gas stream20 is enriched in methane and is depleted in carbon dioxide. On theother hand, the first waste gas stream 22 is relatively depleted inmethane and enriched in carbon dioxide as compared to the crude landfillgas stream 14.

For example, the amount of methane in the crude landfill gas stream 14may range from about 45 to about 55 mole percent and the amount ofcarbon dioxide may range from about 20 to about 40 mole percent. Theamount of methane in the intermediate landfill gas stream 20 may rangefrom about 70 to about 75 mole percent and the amount of carbon dioxidemay range from about 0.5 to about 1.5 mole percent. The amount ofmethane in the first waste gas stream 22 may range from about 8 to about12 mole percent and the amount of carbon dioxide may range from about 75to about 85 mole percent.

The membrane separation unit 16 is typically skid mounted and ispreferably sized in accordance with the flow rate of the crude landfillgas stream 14 to be treated. For example, a membrane separation unit 16having a size of about 10 feet by about 35 feet (representing the sizeof the skid upon which the membranes and the associated mechanicalcomponents are mounted) may be suitable to treat a crude landfill gasstream 14 having a flow rate of about 2 million cubic feet per day.

In general, the membrane separation unit 16 removes carbon dioxide gasfrom the crude landfill gas stream 14 by use of a bundle of hollowfibers disposed within the membrane separation unit 16. The surface ofeach hollow fiber is made from a membrane material which may be readilypermeated by carbon dioxide gas, oxygen gas, water vapor, and VOC gases.The membrane material is substantially less permeable to methane. As thecrude landfill gas stream 14 flows through the membrane separation unit16, it initially travels inside of the hollow fiber bundles. However,substantial amounts of carbon dioxide gas, oxygen gas, water vapor, andVOC gases permeate the membranes of the fibers. This permeate gas may becollected as the first waste gas stream 22. The methane and other gaseswhich do not permeate through the membranes of the fibers may beseparately collected as the intermediate landfill gas stream 20.

During this process, the membrane separation unit 16 is typicallyoperated at an inlet pressure of from about 175 to about 225 psi and ata temperature of from about 100 to about 135° F.

An example of a suitable membrane separation unit 16 for use inaccordance with the present disclosure is the BIOGAZ membrane system,available from Air Liquide—Medal of Newport, Del.

After the intermediate landfill gas stream 20 has been treated in themembrane separation unit 16, it then passes through a conduit and entersthe pressure swing adsorption unit 18 which is in flow communicationwith the membrane separation unit 16. In general, the pressure swingadsorption unit 18 is used to remove additional carbon dioxide from thelandfill gas stream as well as nitrogen. The pressure swing adsorptionunit 18 may also remove oxygen from the landfill gas stream as well. Inso doing, the pressure swing adsorption unit 18 separates theintermediate landfill gas stream 20 into at least two gas streams, thefinal landfill gas stream 24 and the second waste gas stream 26.Relative to the intermediate landfill gas stream 20, the final landfillgas stream 24 is enriched in methane and is depleted in carbon dioxideand nitrogen. On the other hand, the second waste gas stream 26 isrelatively depleted in methane and enriched in carbon dioxide andnitrogen as compared to the intermediate landfill gas stream 20.

For example, as noted above the amount of methane in the intermediatelandfill gas stream 20 may range from about 70 to about 75 mole percent,the amount of carbon dioxide may range from about 5 to about 1.5 molepercent, and the amount of nitrogen may range from about 25 to about 30mole percent.

The amount of methane in the final landfill gas stream 24 may range fromabout 95 to about 97 mole percent, the amount of carbon dioxide mayrange from about 0 to about 5 mole percent, and the amount of nitrogenmay range from about 3 to about 4 mole percent. The amount of methane inthe second waste gas stream 26 may range from about 20 to about 25 molepercent the amount of carbon dioxide may range from about 3 to about 5mole percent, and the amount of nitrogen may range from about 70 toabout 75 mole percent.

The pressure swing adsorption unit 18 is typically skid mounted and ispreferably sized in accordance with the flow rate of the intermediatelandfill gas stream 20 to be treated. For example, a pressure swingadsorption unit 18 having a size of about 8 feet by about 35 feet(representing the size of the skid upon which the unit and theassociated mechanical components are mounted) may be suitable to treatan intermediate landfill gas stream 20 having a flow rate of about 1.25million cubic feet per day.

The pressure swing adsorption unit 18 removes carbon dioxide andnitrogen gases from the intermediate landfill gas stream 20 using aplurality of fixed bed adsorption vessels. The surfaces of the adsorbenthave a multitude of microscopic pores which are sized such thatrelatively smaller molecules, such as carbon dioxide and diatomicnitrogen, may enter into the pores and become trapped therein: however,relatively large molecules, including in particular methane, are toolarge to enter into the pores or to become trapped. The methane thuspasses through the adsorption vessel into the final landfill gas stream24.

When the adsorbent bed eventually becomes saturated with carbon dioxideand/or nitrogen, the bed may be regenerated using vacuum and a flow of apurge gas. The gases collected from the regeneration process may becollected to provide the second waste gas steam.

The fixed bed adsorption vessels are typically each configured so as tobe connected to the membrane separation unit 16 in a parallelarrangement. In this manner, one adsorption vessel may be regeneratedwhile at least one other adsorption vessel remains in operation toprocess the intermediate landfill gas stream 20.

The pressure swing adsorption unit 18 is typically operated at an inletpressure of from about 130 to about 150 psig and at a temperature offrom about 100° F. to about 150° F.

An example of a suitable pressure swing adsorption unit 18 for use inaccordance with the present disclosure is the MOLECULAR GATE pressureswing adsorption system, available from Guild Associates, Inc. ofDublin, Ohio.

In some embodiments according to the present disclosure, the system mayalso include the thermal oxidizer unit 30. In particular, if the initialcrude landfill gas stream 14 includes a substantial amount of VOCs,these VOCs may also be present in the first waste gas stream 22 and/orthe second waste gas stream 26. In these instances, it is preferred thatthe system 10 include the thermal oxidizer 30 which is in flowcommunication with the membrane separation unit 16 and/or the pressureswing adsorption unit 18 so as to receive the first and/or second wastegas streams 22, 26 and substantially destroy the VOCs therein.

In general, the use of the thermal oxidizer unit 30 in the system ispreferred if the concentration of VOCs in either of the first and secondwaste gas streams 22, 26 is sufficiently high to necessitate treatmentprior to release of the gas into the atmosphere. The thermal oxidizerunit 30 preferably destroys at least about 98 percent of the VOCspresent in the waste gas streams. With the level of VOCs so reduced, thefirst and/or second waste gas streams 22, 26 may be released into theatmosphere.

The thermal oxidizer unit 30 is preferably sized in accordance with theflow rate of the waste gas stream or streams to be treated. For example,a thermal oxidizer unit 30 having a width of about 4.5 feet and a heightof about 35 feet may be suitable to treat a waste gas stream having aflow rate of about 1.25 million cubic feet of waste gases per day.

The thermal oxidizer unit 30 is typically operated at a pressure of fromabout 100 to about 140 psi and at a system design temperature at whichthe desired VOC destruction ratios are achieved. Typically, thetemperature is at least about 1600° F.

An example of a suitable thermal oxidizer unit 30 for use in accordancewith the present disclosure is available from MRW Technologies. Inc. ofJenks, Okla.

In certain embodiments according to the present disclosure, the system10 also includes one or more recycle conduits for recombining gases withthe crude landfill gas stream 14. For instance, the system 10 mayinclude a first recycle conduit in flow communication with at least themembrane separation unit 16. According to this embodiment, the membraneseparation unit 16 separates the crude landfill gas stream 14 into atleast three streams, a first recycle gas stream 32 and theaforementioned intermediate landfill gas stream 20 and first waste gasstream 22. The first recycle gas stream 32 is then recombined with thecrude landfill gas stream 14 via the recycle conduit. In certainembodiments, this first recycle gas stream 32 is preferably from about30 to about 40 mole percent of the crude landfill gas stream 14 enteringthe membrane separation unit 16.

In certain embodiments, the system 10 may include a second recycleconduit in flow communication with at least the pressure swingadsorption unit 18. According to this embodiment, the pressure swingadsorption unit 18 separates the intermediate landfill gas stream 20into at least three streams, a second recycle gas stream 34 and theaforementioned final landfill gas stream 24 and second waste gas stream26. The second recycle gas stream 34 is then recombined with the crudelandfill gas stream 14 via the recycle conduit. In certain embodiments,this second recycle gas stream 34 is preferably from about 17 to about22 mole percent of the intermediate landfill gas stream 20 entering thepressure swing adsorption unit 18.

As noted above, treatment of the crude landfill gas stream 14 accordingto the method and system of the present disclosure provides the finallandfill gas steam 24 which is highly enriched in methane and hasrelatively low levels of other gas components. The final landfill gasstream 24 is preferably composed of at least about 96 mole percentmethane. Preferably the final landfill gas stream 24 also includes nomore than about 1 mole percent carbon dioxide, no more than about 4 molepercent nitrogen, no more than about 1 mole percent oxygen, no more thanabout seven pounds of entrained water per million cubic feet of gas, andno more than about 0.25 grains of hydrogen sulfide per hundred cubicfeet of gas. The final landfill gas stream also preferably has an energyvalue of about 970 British Thermal Units (BTUs) per cubic foot of gas.Advantageously, in some embodiments of the present disclosure, the finallandfill gas stream 24 may be provided as a “pipeline quality” methaneproduct in compliance with the particularized requirements of the tariffor schedule applicable to the pipeline into which the methane productwill be delivered. That is, the level of other gas components in the gasis sufficiently low so that the methane stream may be introduced intoand transported through commercial natural gas pipelines and eventuallysold as fungible with natural gas.

The following nonlimiting examples illustrate various additional aspectsof the disclosure. Unless otherwise indicated, temperatures are indegrees Celsius and percentages are by weight based on the dry weight ofthe formulation.

PROPHETIC EXAMPLE

In this example, the treatment of a crude landfill gas stream accordingto one embodiment of the present disclosure is simulatedcomputationally. According to the simulation, a crude landfill gasstream is recovered from a landfill via a plurality of extraction wells.The initial composition of the crude landfill gas stream is about 45.1mole percent methane, 36.07 mole percent carbon dioxide, 17.5 molepercent nitrogen, 1.33 mole percent oxygen, and a saturation amount ofwater vapor. The flow rate of the crude landfill gas stream isapproximately 2.0 million cubic feet per day.

The crude landfill gas stream is compressed to a pressure ofapproximately 215 psig and then introduced into a BIOGAZ membranesystem, from Air Liquide—Medal. After processing in the membraneseparation unit, approximately 1.137 million cubic feet per day of thegas is collected as an intermediate landfill gas stream. The compositionof the intermediate landfill gas stream is determined to be about 71.2mole percent methane, 1 mole percent carbon dioxide, 27.15 mole percentnitrogen, and 0.62 mole percent oxygen. The intermediate landfill gasstream is substantially dry. i.e. no measurable water vapor. Theremainder of the gas from the membrane separation unit is collected as afirst waste gas stream.

The intermediate landfill gas stream is then treated in a MOLECULAR GATEpressure swing adsorption system, from Guild Associates. Inc. Afterprocessing in the pressure swing adsorption unit approximately 0.72million cubic feet per day of a final landfill gas stream is collected.The composition of the final landfill gas stream is determined to beabout 96.19 mole percent methane, 3.00 mole percent nitrogen, and 0.81mole percent oxygen. The final landfill gas stream includessubstantially no carbon dioxide or water vapor. Based on itscomposition, the energy value of the final gas stream is determined tobe about 972 BTUs per cubic foot. The final landfill gas stream thusobtained may be sold as a methane product which is equivalent tocommercial natural gas. A second waste gas stream is also collected fromthe pressure swing adsorption unit.

The first and second waste gas streams are combined into 1.28 millioncubic feet per day gas stream having a composition of approximately 15mole percent methane, 57 mole percent carbon dioxide, 26 mole percentnitrogen, and 2 mole percent oxygen. This gas stream is then treated ina thermal oxidizer in order to destroy any VOCs therein.

The foregoing description of preferred embodiments for this inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Obvious modifications or variations are possible inlight of the above teachings. The embodiments are chosen and describedin an effort to provide the best illustrations of the principles of theinvention and its practical application, and to thereby enable one ofordinary skill in the art to utilize the invention in variousembodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the invention as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

1. A method for separating and purifying a landfill gas stream, themethod comprising the steps of: collecting a crude landfill gas streamwhich includes at least methane, carbon dioxide, oxygen, and nitrogengases, separating the crude landfill gas stream into at least anintermediate landfill gas stream and a first waste gas stream using amembrane separation unit, wherein the intermediate landfill gas streamis enriched in methane and depleted in carbon dioxide relative to thecrude landfill gas stream: and separating the intermediate landfill gasstream into at least a final landfill gas stream and a second waste gasstream using a pressure swing adsorption separation unit, wherein thefinal landfill gas stream is enriched in methane and depleted in carbondioxide, oxygen, and nitrogen relative to the intermediate landfill gasstream.
 2. The method of claim 1, wherein the crude landfill gas streamcomprises from about 45 to about 55 mole percent methane, from about 20to about 40 mole percent carbon dioxide, and from about 5 to about 20mole percent nitrogen.
 3. The method of claim 1, wherein the finallandfill gas stream comprises at least about 96 mole percent methane, nomore than about 1 mole percent carbon dioxide, and no more than about 4mole percent nitrogen.
 4. The method of claim
 1. Wherein the first wastegas stream comprises volatile organic compounds and wherein the methodfurther comprises a step of treating the first waste gas stream in athermal oxidizer unit to substantially destroy the volatile organiccompounds therein so that the first waste gas stream may be releasedinto the atmosphere.
 5. The method of claim 1, wherein the second wastegas stream comprises volatile organic compounds and wherein the methodfurther comprises a step of treating the second waste gas stream in athermal oxidizer unit to substantially destroy the volatile organiccompounds therein so that the second waste gas stream may be releasedinto the atmosphere.
 6. The method of claim 1, wherein at least aportion of the crude landfill gas stream is separated into a firstrecycle gas stream using the membrane separation unit and wherein thefirst recycle gas stream is recombined with the crude landfill gasstream.
 7. The method of claim 6, wherein the first recycle gas streamcomprises from about 30 to about 40 mole percent of the crude landfillgas stream.
 8. The method of claim 1, wherein at least a portion of theintermediate landfill gas stream is separated into a second recycle gasstream using the pressure swing adsorption separation unit and whereinthe second recycle gas stream is recombined with the crude landfill gasstream.
 9. The method of claim 8, wherein the second recycle gas streamcomprises from about 17 to about 22 mole percent of the intermediatelandfill gas stream.
 10. The method of claim 1, wherein the crudelandfill gas stream further includes hydrogen sulfide gas and whereinthe method further comprising a step of removing at least a portion ofthe hydrogen sulfide gas from the crude landfill gas stream before thecrude landfill gas stream is separated using the membrane separationunit.
 11. The method of claim 1, wherein the crude landfill gas streamfurther includes oxygen, water vapor, and volatile organic compounds andwherein the final landfill gas stream is depleted in oxygen, watervapor, and volatile organic compounds relative to the crude landfill gasstream.
 12. A system for separating and purifying a landfill gas streamcomprising: a feed gas conduit in flow communication with a crudelandfill gas stream which includes at least methane, carbon dioxide,oxygen, and nitrogen gases: a membrane separation unit in flowcommunication with the feed gas conduit for separating the crudelandfill gas stream into at least an intermediate landfill gas streamand a first waste gas stream, wherein the intermediate landfill gasstream is enriched in methane and depleted in carbon dioxide relative tothe crude landfill gas stream; and a pressure swing adsorptionseparation unit in flow communication with the membrane separation unitfor receiving the intermediate landfill gas stream therefrom andseparating the intermediate landfill gas stream into at least a finallandfill gas stream and a second waste gas stream, wherein the finallandfill gas stream is enriched in methane and depleted in carbondioxide, oxygen, and nitrogen relative to the intermediate landfill gasstream.
 13. The system of claim 12, wherein the crude landfill gasstream comprises from about 45 to about 55 mole percent methane, fromabout 20 to about 40 mole percent carbon dioxide, and from about 5 toabout 20 mole percent nitrogen.
 14. The system of claim 12, wherein thefinal landfill gas stream comprises at least about 96 mole percentmethane, no more than about 1 mole percent carbon dioxide, and no morethan about 4 mole percent nitrogen.
 15. The system of claim 12, furthercomprising a thermal oxidizer unit in flow communication with themembrane separation unit for receiving the first waste gas stream,wherein the first waste gas stream comprises volatile organic compoundsand wherein the thermal oxidizer unit substantially destroy the volatileorganic compounds in the first waste gas stream so that the first wastegas stream may be released into the atmosphere
 16. The system of claim12, further comprising a thermal oxidizer unit in flow communicationwith the pressure swing adsorption unit for receiving the second wastegas stream, wherein the second waste gas stream comprises volatileorganic compounds and wherein the thermal oxidizer unit substantiallydestroy the volatile organic compounds in the second waste gas stream sothat the second waste gas stream may be released into the atmosphere 17.The system of claim 12, further comprising a recycle conduit in flowcommunication with the membrane separation unit, wherein at least aportion of the crude landfill gas stream is separated into a firstrecycle gas stream using the membrane separation unit and wherein thefirst recycle gas stream is recombined with the crude landfill gasstream via the recycle conduit.
 18. The system of claim 17, wherein thefirst recycle gas stream comprises from about 30 to about 40 molepercent of the crude landfill gas stream.
 19. The system of claim 12,further comprising a recycle conduit in flow communication with thepressure swing adsorption unit, wherein at least a portion of theintermediate landfill gas stream is separated into a second recycle gasstream using the pressure swing adsorption separation unit and whereinthe second recycle gas stream is recombined with the crude landfill gasstream via the recycle conduit.
 20. The system of claim 19, wherein thesecond recycle gas stream comprises from about 17 to about 22 molepercent of the intermediate landfill gas stream.
 21. The system of claim12, wherein the crude landfill gas stream further includes hydrogensulfide gas and wherein the system further comprises a hydrogen sulfideseparation unit in flow communication with the crude landfill gas streamfor removing at least a portion of the hydrogen sulfide gas from thecrude landfill gas stream before the crude landfill gas stream isseparated using the membrane separation unit.
 22. The system of claim12, wherein the crude landfill gas stream further includes oxygen watervapor, and volatile organic compounds and wherein the final landfill gasstream is depleted in oxygen, water vapor, and volatile organiccompounds relative to the crude landfill gas stream.